The overall goal of this project is to test both what is familial and what is specific about the idiopathic autism phenotype (IAP) at two levels of analysis: the symptom and neuropsychological levels. Previous research has identified social and/or language abnormalities in non-autistic first- degree relatives of probands with autism, which is referred to her as the Familial Autism Phenotype (FAP). This Project builds on that previous work and will specify in more detail the nature of the language abnormality in the FAP. However, neither the full range of symptoms nor all the underlying neuropsychological deficits founds in the IAP have been tested in non-autistic relatives, so that both the boundaries of the FAP and its similarity to the IAP remain unclear. It is also unclear which aspects of the FAP and the IAP are specific to autism and not found in other, even closely related developmental disabilities. This project will attempt to close these gaps in existing knowledge by comparing first degree relatives of autistic sib pairs with 1) first degree relatives of Down syndrome (DS) children, 2) females with fragile X syndrome (FXS) from Project III.